Known piezoelectric/electrostrictive film elements of this kind include actuators which utilize flexural displacements of their thin-walled diaphragm portions, and sensors (e.g., microphones and viscosity sensors) for detecting characteristics (e.g., fluidal characteristic, sound pressure, very small weight, and acceleration) of media in the proximity of the thin-walled diaphragm portions.
Japanese Patent Application Laid-Open (kokai) Nos. H8-201265 (Patent Document 1), 2002-261347 (Patent Document 2), and 2005-322890 (Patent Document 3) disclose piezoelectric/electrostrictive film elements which serve as the above-mentioned sensors. The piezoelectric/electrostrictive film sensors are configured to be able to utilize the correlation between the amplitude of a piezoelectric/electrostrictive film vibrator and the viscous resistance of fluid in contact with the vibrator and to measure characteristics, such as density, concentration, and viscosity, of the fluid.
Specifically, in the piezoelectric/electrostrictive film sensors, when the vibrator is vibrated in the presence of the fluid, the vibrator is subjected to a dynamic resistance caused by viscosity of the fluid. In the meantime, a mechanical vibrating condition of the vibrator can be replaced with an equivalent electrical circuit. Accordingly, on the basis of viscous resistance to which the vibrator is subjected, an electrical constant of an equivalent circuit of a piezoelectric/electrostrictive film serving as the vibrator varies. By detecting variation of the electrical constant, characteristics, such as viscosity, density, and concentration, of the fluid can be measured.
Fluid whose characteristics can be measured by such the piezoelectric/electrostrictive film sensors encompasses liquid and gas. Examples of liquid serving as an object of measurement include, as a matter of course, a single-component liquid formed solely of a main medium selected from among, for example, water, alcohol, oil, etc. Also, the examples of the object-of-measurement liquid can include a liquid substance (slurry, paste, or the like) formed in such a manner that another medium that is soluble, lightly soluble, or insoluble in such a main medium is added to (dissolved in, mixed with, diffused in, or suspended in) the main medium.
Also, examples of the above-mentioned electrical constant include loss factor, phase, resistance, reactance, conductance, susceptance, inductance, and capacitance. Particularly, phase, or loss factor, which has a single maximal or minimal variation point in the vicinity of the resonance frequency of an equivalent circuit, is preferably used. This enables measurement of density and concentration of the fluid in addition to viscosity of the fluid. For example, the concentration of sulfuric acid in an aqueous solution of sulfuric acid can be measured. In addition to the above-mentioned electrical constants, variation of resonance frequency can be used as an index for detecting variation in vibrating condition, so long as no particular problem arises in terms of accuracy of measurement, and durability.